Technical Field
This disclosure relates to communication systems supporting simultaneous transmit and receive, co-existent communication devices, systems requiring enhanced isolation between certain ports or blocks, and other systems where signal cancellations are achieved through hybrid couplers.
Description of Related Art
In a frequency division duplex (FDD) radio communication system, the transmitter (TX) and the receiver (RX) of the same radio may operate simultaneously, but, in two different frequency bands, fTX for TX and fRX for RX. In such scenarios, an important performance metric for the radio communication system may be the degree of “isolation” between the TX and the RX that are operating simultaneously. The isolation can be particularly important within the TX frequency band and within the RX frequency band. Any TX leakage that reaches the RX may interfere significantly with the receiver.
It should be noted that, for simplicity and consistent with common notations, a singular frequency (such as fTX & fRX) is used herein to designate a frequency band that includes many (infinite) frequencies. These singular frequencies often refer to a frequency within the frequency band such as the center of the band or edge of the band. This notation should not be construed to limit the teachings of this disclosure in any way.
On the other hand, many platforms include several communication devices at close proximity. Examples may include commercial or military platforms, such as handheld or portable platforms, that include various voice and data communication transceivers as well as wireless positioning solutions. For instance, modern smartphones include cellular phone transceivers, Bluetooth transceivers, WiFi transceivers, Global Positioning System (GPS) receivers, and in some instances radio or television receivers. In these platforms, various communication devices may operate at the same time, resulting in unwanted interference among them. In such co-existence scenarios, high isolation between various simultaneously-operating communication devices may be needed to ensure proper operation.
The minimum required isolation may depend on the application and the scenario. For example, in a typical commercial FDD radio standard, TX to RX isolation of 50 dB or more may be required in the TX and the RX frequency bands. Without adequate isolation, the aggressor TX signal may significantly deteriorate the sensitivity of the victim RX and ultimately prevent its proper operation. In a co-existence radio, the isolation between the platforms may need to be even higher. For instance, the frequencies allocated to WiFi standards may be very close to the frequencies allocated to cellular phones. The high power level of a transmitter in a WiFi transceiver may degrade the receiving performance of a cellular receiver and vice versa.
One approach to providing TX-RX isolation is to use a frequency duplexer. The frequency duplexer is a three port electrical network. One port is typically connected to the antenna (ANT), one port is typically connected to the TX output, and one port is typically connected to the RX input. There may be other components, such as impedance matching networks or filters or coupling/decoupling components, between the duplexer ports and ANT, RX, and TX ports.
A common challenge in duplexer design is to achieve low insertion loss from TX to ANT and from ANT to RX, while providing high isolation from TX to RX. However, meeting this requirement may require technologies that offer high quality factor (Q), or low loss, components and resonators. These technologies, such as those based on bulk acoustic wave (BAW), are often more expensive or bulky compared with technologies that do not offer high-Q components and resonators.
Another approach to enhancing isolation between TX and RX is to generate a cancellation signal that fully or partially matches the amplitude of the leakage signal from the aggressor TX to the victim RX, but with a negative sign. The cancellation signal is then combined with the leakage signal (subtraction) resulting in an enhanced isolation between TX and RX.
Examples of cancellation networks utilizing quadrature hybrids and other components to ensure an acceptable amount of isolation among various ports are disclosed in U.S. Pat. Nos. 2,561,212, 7,123,883, and 7,623,005, and U.S. pre-grant publications 2013/0201880, 2013/0201881, and 2013/0321097.
A variety of non-idealities, such as component mismatches, path imbalances, finite component isolations, impedance mismatches, and varying antenna mismatch, however, can diminish the effectiveness of the cancellation and thus the degree of isolation that is achieved, as well as the amount of signal return losses at an impedance mismatched port.
A common approach to improve the performance of a hybrid based duplexer in the presence of antenna impedance mismatch is to include an antenna tuner prior to the antenna. U.S. pre-grant publication 2013/0201880 discloses such an approach in the context of a tunable duplexer. U.S. pre-grant publication 2013/0321097 discloses an alternative approach where tunable loads and 90 degree phase shifters are introduced to a hybrid based duplexer scheme to improve the isolation in the presence of antenna impedance mismatch.
Approaches towards mitigation of the antenna impedance mismatch in hybrid based duplexers can require components, such as phase shifters, that may not be realizable in compact, low loss, and cost efficient ways, especially when the duplexer should cover a wide range of frequencies. Many communication systems, including those for cellular phone and wireless connectivity standards, may need to support large contiguous or noncontiguous frequency bands to increase the data-rate, diversity, or robustness.